1. Field of the Invention
The present invention relates to a slot curtain coating apparatus and a slot curtain coating method, in both of which a coating liquid ejected from a slit is applied on a continuously running web, while so guiding the coating liquid in the form of a curtain with curtain edge guides.
2. Description of the Related Art
As a coating film is formed in the slot curtain coating method, a portion where a flow of a coating liquid is slow, which is called a boundary layer, is created adjacent to both edge guides of a curtain film, when the coating liquid free falls. Due to this difference in the flow rate, a difference in surface tension is generated. A phenomenon that parts of the coating liquid adjacent to the both edges of the curtain film move towards the center is caused by Marangoni flow that is generated due to the surface tension difference. As a coating film is formed with hitting the coating liquid onto a continuously running web in the aforementioned state, there is a problem that thin film parts 20a are formed at edges of the coating film 20 with respect to the width direction thereof, as illustrated in FIG. 1.
For example, disclosed is a method for preventing a formation of a boundary layer in a curtain film (Japanese Patent Application Laid-Open (JP-A) No. 2008-520753). This method involves providing a porous material to a surface of an edge guide at which a curtain film is flown down, and ejecting auxiliary water from the porous material to cover the porous material with the auxiliary water, to reduce friction resistance of the curtain film with the surface of the edge guide at which the curtain film flows down, to prevent reduction in a flow-down speed of the coating liquid adjacent to the edge guides, to thereby prevent a formation of a boundary layer.
This method has an effect when a viscosity of the coating liquid is relatively low, such as a several tens cp. As a coating liquid having a high viscosity is used, however, a thickness of the film becomes uneven due to an influence of a boundary layer. Moreover, clogging of the porous material is caused with the coating liquid, and ejection of the edge guide auxiliary water becomes uneven, causing coating defects, such as uneven film thickness.
A method for solving the aforementioned clogging is disclosed in U.S. Pat. No. 7,081,163. Disclosed is a technique where a metal surface is provided as an auxiliary ater flow-down surface of the edge guide, and auxiliary water is ejected from an outlet provided in the metal surface.
The aforementioned technique however has a problem that it is hard to uniformly eject the edge guide auxiliary water, as it employs a structure where the edge guide auxiliary water is directly flown from the outlet, a problem that a curtain film is not stabilized, as the auxiliary water does not linearly fall and the auxiliary water flow-down surface is a flat surface, and a problem that a curtain film is swung by a disturbance of wind.
Moreover, disclosed in JP-A No. 2001-46939 in order to solve the problem that it is difficult to uniformly eject the edge guide auxiliary water is a method containing providing a slit portion and a manifold portion that is a liquid retention section, through which the edge guide auxiliary water pass until being ejected from the outlet. In this method, however, the ejection direction of the edge guide auxiliary water is different from the flow down direction of the coating liquid, and therefore the coating liquid cannot be sufficiently accelerated.
As for another method, disclosed is a method, in which a formation of a boundary layer is prevented adjacent to both edges of the curtain film by ejecting the edge guide auxiliary water to the edge guide in the flow-down direction of the coating liquid (JP-A No. 01-199668). In this method, however, acceleration of the curtain film with the edge guide auxiliary water is not sufficient, and therefore a boundary layer cannot be completely eliminated.
Moreover, it is also disclosed in JP-A No. 06-218314 that the edge guide auxiliary water is ejected to the edge guide in the flow-down direction of the coating liquid. This further discloses a size of a flow channel of the auxiliary water inside the edge guide. However, this disclosure does not include a speed of the edge guide auxiliary water. With regard to the size of the flow channel, the flow channel length and the radius R of the curved portion of the flow channel just before the outlet are disclosed. With such sizes, there is a significant difference to an obtainable effect of the present invention.
Furthermore, as a coating liquid having a relatively high viscosity and having a strong shear viscosity reducing effect, such as an adhesive, is used, a thickening effect of the slot curtain film due to an influence of a boundary layer increases.
Similarly to JP-A No. 06-218314, the edge guide auxiliary water is ejected in the curtain flow down direction in JP-A No. 2004-105960. However, there is no description about a radius R of the curved portion of the flow channel just before the outlet therein. Based upon the sizes of the flow channel of the auxiliary water and the outlet, or the size of the flow-down surface of the auxiliary water, an effect of reducing a boundary layer is not large with a coating liquid having a relatively high viscosity, and a strong shear viscosity educing effect, such as an adhesive.
Moreover, disclosed is an ejection speed of an edge guide auxiliary water at which an effect of accelerating a coating liquid is exhibited, a boundary layer is prevented so that an uneven thickness of a curtain film can be prevented with a coating liquid having a relatively high viscosity and a strong shear viscosity reducing effect, such as an adhesive (JP-A No. 2011-78966). In this disclosure, also disclosed are edge guides having resistance to disturbances, such as wind, i.e., a strong aligning effect, as a coating liquid having a relatively high surface tension (35 mN/m or greater) is used.
In the case where a coating liquid having a low surface tension (less than 35 mN/m) is used, disclosed are edge guide having resistance to disturbances, such as wind (JP-A No. 2012-35210).
An influence of a boundary layer is inhibited by flowing an auxiliary water along a surface of an edge guide at which a curtain film falls down, but there is no auxiliary water from the upper plane of the edge guide to the auxiliary water outlet. In the case where a coating liquid having a relatively high viscosity and strong shear viscosity reducing effect is applied, therefore, a boundary layer is easily generated between the aforementioned areas, and an influence of the boundary layer is caused, which have been found from the studies conducted by the present inventors.
The present inventors have come to an insight that an influence of a boundary layer adjacent to edge guides can be inhibited by reducing a distance between the top plane of the edge guide and the auxiliary water outlet. To reduce the distance between the top plane of the edge guide and the auxiliary water outlet, however, is not simple, as it is closely related to a flow channel for introducing auxiliary water.
In fact, a distance h from the top plane of the edge guide to the auxiliary water outlet is determined with h=T+R+S, which is a sum of a thickness T of the thinnest part of the upper part of the curved portion, a curvature radius R of the lower part of the curved portion from the straight-line portion to the auxiliary water outlet, and a thickness S of the slit portion, as illustrated in FIG. 2.
There is a problem of rigidity in order to reduce T, which is a thickness from the top plane of the edge guide to the slit portion. As a downside caused by reducing the curvature radius R of the lower part of the curved portion from the slit portion to the auxiliary water outlet, there is a problem that a flow in a flow channel in the curved portion becomes fast due to a centrifugal force, to thereby generate a disturbance in the flow. As a downside caused by reducing the thickness S of the slit portion, there is a problem that Reynolds number becomes large and a turbulent flow is caused.